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This file is a user guide to the GNU assembler as version
${BFD_VERSION}.
This document is distributed under the terms of the GNU Free Documentation License. A copy of the license is included in the section entitled "GNU Free Documentation License".
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Here is a brief summary of how to invoke as. For details,
see section Comand-Line Options.
as [ -a[cdhlns][=file] ] [ -D ] [ --defsym sym=val ] [ -f ] [ --gstabs ] [ --gdwarf2 ] [ --help ] [ -I dir ] [ -J ] [ -K ] [ -L ] [ --keep-locals ] [ -o objfile ] [ -R ] [ --statistics ] [ -v ] [ -version ] [ --version ] [ -W ] [ --warn ] [ --fatal-warnings ] [ -w ] [ -x ] [ -Z ] [ --target-help ] [ -m[arm]1 | -m[arm]2 | -m[arm]250 | -m[arm]3 | -m[arm]6 | -m[arm]60 | -m[arm]600 | -m[arm]610 | -m[arm]620 | -m[arm]7[t][[d]m[i]][fe] | -m[arm]70 | -m[arm]700 | -m[arm]710[c] | -m[arm]7100 | -m[arm]7500 | -m[arm]8 | -m[arm]810 | -m[arm]9 | -m[arm]920 | -m[arm]920t | -m[arm]9tdmi | -mstrongarm | -mstrongarm110 | -mstrongarm1100 ] [ -m[arm]v2 | -m[arm]v2a | -m[arm]v3 | -m[arm]v3m | -m[arm]v4 | -m[arm]v4t | -m[arm]v5 | -[arm]v5t | -[arm]v5te ] [ -mthumb | -mall ] [ -mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu ] [ -EB | -EL ] [ -mapcs-32 | -mapcs-26 | -mapcs-float | -mapcs-reentrant ] [ -mthumb-interwork ] [ -moabi ] [ -k ] [ -Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite -Av8plus | -Av8plusa | -Av9 | -Av9a ] [ -xarch=v8plus | -xarch=v8plusa ] [ -bump ] [ -32 | -64 ] [ -m68hc11 | -m68hc12 ] [ --force-long-branchs ] [ --short-branchs ] [ --strict-direct-mode ] [ --print-insn-syntax ] [ --print-opcodes ] [ --generate-example ] [ -nocpp ] [ -EL ] [ -EB ] [ -G num ] [ -mcpu=CPU ] [ -mips1 ] [ -mips2 ] [ -mips3 ] [ -mips4 ] [ -mips5 ] [ -mips32 ] [ -mips64 ] [ -m4650 ] [ -no-m4650 ] [ --trap ] [ --break ] [ --emulation=name ] [ -- | files ... ] |
-a[cdhlmns]
-ac
-ad
-ah
-al
-am
-an
-as
=file
You may combine these options; for example, use `-aln' for assembly listing without forms processing. The `=file' option, if used, must be the last one. By itself, `-a' defaults to `-ahls'.
-D
--defsym sym=value
-f
--gstabs
--gdwarf2
--help
--target-help
-I dir
.include directives.
-J
-K
-L
--keep-locals
-o objfile
as objfile.
-R
--statistics
--strip-local-absolute
-v
-version
as version.
--version
as version and exit.
-W
--no-warn
--fatal-warnings
--warn
-w
-x
-Z
-- | files ...
The following options are available when as is configured for the ARM processor family.
-m[arm][1|2|3|6|7|8|9][...]
-m[arm]v[2|2a|3|3m|4|4t|5|5t]
-mthumb | -mall
-mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu
-mapcs-32 | -mapcs-26 | -mapcs-float | -mapcs-reentrant | -moabi
-EB | -EL
-mthumb-interwork
-k
The following options are available when as is configured for the Motorola 68HC11 or 68HC12 series.
-m68hc11 | -m68hc12
--force-long-branchs
-S | --short-branchs
--strict-direct-mode
--print-insn-syntax
--print-opcodes
--generate-example
as.
The following options are available when as is configured
for the SPARC architecture:
-Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite
-Av8plus | -Av8plusa | -Av9 | -Av9a
`-Av8plus' and `-Av8plusa' select a 32 bit environment. `-Av9' and `-Av9a' select a 64 bit environment.
`-Av8plusa' and `-Av9a' enable the SPARC V9 instruction set with UltraSPARC extensions.
-xarch=v8plus | -xarch=v8plusa
-bump
The following options are available when as is configured for a MIPS processor.
-G num
gp register. It is only accepted for targets that
use ECOFF format, such as a DECstation running Ultrix. The default value is 8.
-EB
-EL
-mips1
-mips2
-mips3
-mips4
-mips32
-m4650
-no-m4650
-mcpu=CPU
--emulation=name
as to emulate as configured
for some other target, in all respects, including output format (choosing
between ELF and ECOFF only), handling of pseudo-opcodes which may generate
debugging information or store symbol table information, and default
endianness. The available configuration names are: `mipsecoff',
`mipself', `mipslecoff', `mipsbecoff', `mipslelf',
`mipsbelf'. The first two do not alter the default endianness from that
of the primary target for which the assembler was configured; the others change
the default to little- or big-endian as indicated by the `b' or `l'
in the name. Using `-EB' or `-EL' will override the endianness
selection in any case.
This option is currently supported only when the primary target
as is configured for is a MIPS ELF or ECOFF target.
Furthermore, the primary target or others specified with
`--enable-targets=...' at configuration time must include support for
the other format, if both are to be available. For example, the Irix 5
configuration includes support for both.
Eventually, this option will support more configurations, with more fine-grained control over the assembler's behavior, and will be supported for more processors.
-nocpp
as ignores this option. It is accepted for compatibility with
the native tools.
--trap
--no-trap
--break
--no-break
1.1 Structure of this Manual 1.2 The GNU Assembler 1.3 Object File Formats 1.4 Command Line 1.5 Input Files 1.6 Output (Object) File 1.7 Error and Warning Messages
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This manual is intended to describe what you need to know to use
GNU as. We cover the syntax expected in source files, including
notation for symbols, constants, and expressions; the directives that
as understands; and of course how to invoke as.
This manual also describes some of the machine-dependent features of various flavors of the assembler.
On the other hand, this manual is not intended as an introduction to programming in assembly language--let alone programming in general! In a similar vein, we make no attempt to introduce the machine architecture; we do not describe the instruction set, standard mnemonics, registers or addressing modes that are standard to a particular architecture. You may want to consult the manufacturer's machine architecture manual for this information.
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GNU as is really a family of assemblers.
If you use (or have used) the GNU assembler on one architecture, you
should find a fairly similar environment when you use it on another
architecture. Each version has much in common with the others,
including object file formats, most assembler directives (often called
pseudo-ops) and assembler syntax.
as is primarily intended to assemble the output of the
GNU C compiler gcc for use by the linker
ld. Nevertheless, we've tried to make as
assemble correctly everything that other assemblers for the same
machine would assemble.
Unlike older assemblers, as is designed to assemble a source
program in one pass of the source file. This has a subtle impact on the
.org directive (see section .org).
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The GNU assembler can be configured to produce several alternative object file formats. For the most part, this does not affect how you write assembly language programs; but directives for debugging symbols are typically different in different file formats. See section Symbol Attributes.
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After the program name as, the command line may contain
options and file names. Options may appear in any order, and may be
before, after, or between file names. The order of file names is
significant.
`--' (two hyphens) by itself names the standard input file
explicitly, as one of the files for as to assemble.
Except for `--' any command line argument that begins with a
hyphen (`-') is an option. Each option changes the behavior of
as. No option changes the way another option works. An
option is a `-' followed by one or more letters; the case of
the letter is important. All options are optional.
Some options expect exactly one file name to follow them. The file name may either immediately follow the option's letter (compatible with older assemblers) or it may be the next command argument (GNU standard). These two command lines are equivalent:
as -o my-object-file.o mumble.s as -omy-object-file.o mumble.s |
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We use the phrase source program, abbreviated source, to
describe the program input to one run of as. The program may
be in one or more files; how the source is partitioned into files
doesn't change the meaning of the source.
The source program is a concatenation of the text in all the files, in the order specified.
Each time you run as it assembles exactly one source
program. The source program is made up of one or more files.
(The standard input is also a file.)
You give as a command line that has zero or more input file
names. The input files are read (from left file name to right). A
command line argument (in any position) that has no special meaning
is taken to be an input file name.
If you give as no file names it attempts to read one input file
from the as standard input, which is normally your terminal. You
may have to type ctl-D to tell as there is no more program
to assemble.
Use `--' if you need to explicitly name the standard input file in your command line.
If the source is empty, as produces a small, empty object
file.
There are two ways of locating a line in the input file (or files) and either may be used in reporting error messages. One way refers to a line number in a physical file; the other refers to a line number in a "logical" file. See section Error and Warning Messages.
Physical files are those files named in the command line given
to as.
Logical files are simply names declared explicitly by assembler
directives; they bear no relation to physical files. Logical file names help
error messages reflect the original source file, when as source
is itself synthesized from other files. as understands the
`#' directives emitted by the gcc preprocessor. See also
.file.
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Every time you run as it produces an output file, which is
your assembly language program translated into numbers. This file
is the object file. Its default name is
a.out.
b.out when as is configured for the Intel 80960.
You can give it another name by using the -o option. Conventionally,
object file names end with `.o'. The default name is used for historical
reasons: older assemblers were capable of assembling self-contained programs
directly into a runnable program. (For some formats, this isn't currently
possible, but it can be done for the a.out format.)
The object file is meant for input to the linker ld. It contains
assembled program code, information to help ld integrate
the assembled program into a runnable file, and (optionally) symbolic
information for the debugger.
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as may write warnings and error messages to the standard error
file (usually your terminal). This should not happen when a compiler
runs as automatically. Warnings report an assumption made so
that as could keep assembling a flawed program; errors report a
grave problem that stops the assembly.
Warning messages have the format
file_name:NNN:Warning Message Text |
(where NNN is a line number). If a logical file name has been given
(see section .file) it is used for the filename, otherwise the name of
the current input file is used. If a logical line number was given
(see section .line)
then it is used to calculate the number printed,
otherwise the actual line in the current source file is printed. The
message text is intended to be self explanatory (in the grand Unix
tradition).
Error messages have the format
file_name:NNN:FATAL:Error Message Text |
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This chapter describes command-line options available in all versions of the GNU assembler; see section 8. Machine Dependent Features, for options specific to particular machine architectures.
If you are invoking as via the GNU C compiler (version 2),
you can use the `-Wa' option to pass arguments through to the assembler.
The assembler arguments must be separated from each other (and the `-Wa')
by commas. For example:
gcc -c -g -O -Wa,-alh,-L file.c |
This passes two options to the assembler: `-alh' (emit a listing to standard output with with high-level and assembly source) and `-L' (retain local symbols in the symbol table).
Usually you do not need to use this `-Wa' mechanism, since many compiler command-line options are automatically passed to the assembler by the compiler. (You can call the GNU compiler driver with the `-v' option to see precisely what options it passes to each compilation pass, including the assembler.)
2.1 Enable Listings: -a[cdhlns]-a[cdhlns] enable listings 2.2 -D-D for compatibility 2.3 Work Faster: -f-f to work faster 2.4 .includesearch path:-Ipath-I for .include search path 2.5 Difference Tables: -K-K for difference tables
2.6 Include Local Labels: -L-L to retain local labels 2.7 Assemble in MRI Compatibility Mode: -M-M or --mri to assemble in MRI compatibility mode 2.8 Dependency tracking: --MD--MD for dependency tracking 2.9 Name the Object File: -o-o to name the object file 2.10 Join Data and Text Sections: -R-R to join data and text sections 2.11 Display Assembly Statistics: --statistics--statistics to see statistics about assembly 2.12 Compatible output: --traditional-format--traditional-format for compatible output 2.13 Announce Version: -v-v to announce version 2.14 Control Warnings: -W,--warn,--no-warn,--fatal-warnings-W, --no-warn, --warn, --fatal-warnings to control warnings 2.15 Generate Object File in Spite of Errors: -Z-Z to make object file even after errors
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-a[cdhlns] These options enable listing output from the assembler. By itself, `-a' requests high-level, assembly, and symbols listing. You can use other letters to select specific options for the list: `-ah' requests a high-level language listing, `-al' requests an output-program assembly listing, and `-as' requests a symbol table listing. High-level listings require that a compiler debugging option like `-g' be used, and that assembly listings (`-al') be requested also.
Use the `-ac' option to omit false conditionals from a listing. Any lines
which are not assembled because of a false .if (or .ifdef, or any
other conditional), or a true .if followed by an .else, will be
omitted from the listing.
Use the `-ad' option to omit debugging directives from the listing.
Once you have specified one of these options, you can further control
listing output and its appearance using the directives .list,
.nolist, .psize, .eject, .title, and
.sbttl.
The `-an' option turns off all forms processing.
If you do not request listing output with one of the `-a' options, the
listing-control directives have no effect.
The letters after `-a' may be combined into one option, e.g., `-aln'.
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-D
This option has no effect whatsoever, but it is accepted to make it more
likely that scripts written for other assemblers also work with
as.
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-f `-f' should only be used when assembling programs written by a (trusted) compiler. `-f' stops the assembler from doing whitespace and comment preprocessing on the input file(s) before assembling them. See section Preprocessing.
Warning: if you use `-f' when the files actually need to be
preprocessed (if they contain comments, for example), as does
not work correctly.
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.include search path: -I path
Use this option to add a path to the list of directories
as searches for files specified in .include
directives (see section .include). You may use -I as
many times as necessary to include a variety of paths. The current
working directory is always searched first; after that, as
searches any `-I' directories in the same order as they were
specified (left to right) on the command line.
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-K
as sometimes alters the code emitted for directives of the form
`.word sym1-sym2'; see section .word.
You can use the `-K' option if you want a warning issued when this
is done.
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-L
Labels beginning with `L' (upper case only) are called local
labels. See section 5.3 Symbol Names. Normally you do not see such labels when
debugging, because they are intended for the use of programs (like
compilers) that compose assembler programs, not for your notice.
Normally both as and ld discard such labels, so you do not
normally debug with them.
This option tells as to retain those `L...' symbols
in the object file. Usually if you do this you also tell the linker
ld to preserve symbols whose names begin with `L'.
By default, a local label is any label beginning with `L', but each target is allowed to redefine the local label prefix.
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-M
The -M or --mri option selects MRI compatibility mode. This
changes the syntax and pseudo-op handling of as to make it
compatible with the ASM68K or the ASM960 (depending upon the
configured target) assembler from Microtec Research. The exact nature of the
MRI syntax will not be documented here; see the MRI manuals for more
information. Note in particular that the handling of macros and macro
arguments is somewhat different. The purpose of this option is to permit
assembling existing MRI assembler code using as.
The MRI compatibility is not complete. Certain operations of the MRI assembler depend upon its object file format, and can not be supported using other object file formats. Supporting these would require enhancing each object file format individually. These are:
The m68k MRI assembler supports common sections which are merged by the linker.
Other object file formats do not support this. as handles
common sections by treating them as a single common symbol. It permits local
symbols to be defined within a common section, but it can not support global
symbols, since it has no way to describe them.
The MRI assemblers support relocations against a negated section address, and relocations which combine the start addresses of two or more sections. These are not support by other object file formats.
END pseudo-op specifying start address
The MRI END pseudo-op permits the specification of a start address.
This is not supported by other object file formats. The start address may
instead be specified using the -e option to the linker, or in a linker
script.
IDNT, .ident and NAME pseudo-ops
The MRI IDNT, .ident and NAME pseudo-ops assign a module
name to the output file. This is not supported by other object file formats.
ORG pseudo-op
The m68k MRI ORG pseudo-op begins an absolute section at a given
address. This differs from the usual as .org pseudo-op,
which changes the location within the current section. Absolute sections are
not supported by other object file formats. The address of a section may be
assigned within a linker script.
There are some other features of the MRI assembler which are not supported by
as, typically either because they are difficult or because they
seem of little consequence. Some of these may be supported in future releases.
EBCDIC strings are not supported.
Packed binary coded decimal is not supported. This means that the DC.P
and DCB.P pseudo-ops are not supported.
FEQU pseudo-op
The m68k FEQU pseudo-op is not supported.
NOOBJ pseudo-op
The m68k NOOBJ pseudo-op is not supported.
OPT branch control options
The m68k OPT branch control options---B, BRS, BRB,
BRL, and BRW---are ignored. as automatically
relaxes all branches, whether forward or backward, to an appropriate size, so
these options serve no purpose.
OPT list control options
The following m68k OPT list control options are ignored: C,
CEX, CL, CRE, E, G, I, M,
MEX, MC, MD, X.
OPT options
The following m68k OPT options are ignored: NEST, O,
OLD, OP, P, PCO, PCR, PCS, R.
OPT D option is default
The m68k OPT D option is the default, unlike the MRI assembler.
OPT NOD may be used to turn it off.
XREF pseudo-op.
The m68k XREF pseudo-op is ignored.
.debug pseudo-op
The i960 .debug pseudo-op is not supported.
.extended pseudo-op
The i960 .extended pseudo-op is not supported.
.list pseudo-op.
The various options of the i960 .list pseudo-op are not supported.
.optimize pseudo-op
The i960 .optimize pseudo-op is not supported.
.output pseudo-op
The i960 .output pseudo-op is not supported.
.setreal pseudo-op
The i960 .setreal pseudo-op is not supported.
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--MD
as can generate a dependency file for the file it creates. This
file consists of a single rule suitable for make describing the
dependencies of the main source file.
The rule is written to the file named in its argument.
This feature is used in the automatic updating of makefiles.
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-o
There is always one object file output when you run as. By
default it has the name
`a.out'.
You use this option (which takes exactly one filename) to give the
object file a different name.
Whatever the object file is called, as overwrites any
existing file of the same name.
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-R
-R tells as to write the object file as if all
data-section data lives in the text section. This is only done at
the very last moment: your binary data are the same, but data
section parts are relocated differently. The data section part of
your object file is zero bytes long because all its bytes are
appended to the text section. (See section Sections and Relocation.)
When you specify -R it would be possible to generate shorter
address displacements (because we do not have to cross between text and
data section). We refrain from doing this simply for compatibility with
older versions of as. In future, -R may work this way.
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--statistics
Use `--statistics' to display two statistics about the resources used by
as: the maximum amount of space allocated during the assembly
(in bytes), and the total execution time taken for the assembly (in CPU
seconds).
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--traditional-format
For some targets, the output of as is different in some ways
from the output of some existing assembler. This switch requests
as to use the traditional format instead.
For example, it disables the exception frame optimizations which
as normally does by default on gcc output.
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-v You can find out what version of as is running by including the option `-v' (which you can also spell as `-version') on the command line.
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-W, --warn, --no-warn, --fatal-warnings
as should never give a warning or error message when
assembling compiler output. But programs written by people often
cause as to give a warning that a particular assumption was
made. All such warnings are directed to the standard error file.
If you use the -W and --no-warn options, no warnings are issued.
This only affects the warning messages: it does not change any particular of
how as assembles your file. Errors, which stop the assembly,
are still reported.
If you use the --fatal-warnings option, as considers
files that generate warnings to be in error.
You can switch these options off again by specifying --warn, which
causes warnings to be output as usual.
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-Z as normally produces no output. If for
some reason you are interested in object file output even after
as gives an error message on your program, use the `-Z'
option. If there are any errors, as continues anyways, and
writes an object file after a final warning message of the form `n
errors, m warnings, generating bad object file.'
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This chapter describes the machine-independent syntax allowed in a
source file. as syntax is similar to what many other
assemblers use; it is inspired by the BSD 4.2
assembler.
3.1 Preprocessing 3.2 Whitespace 3.3 Comments 3.4 Symbols 3.5 Statements 3.6 Constants
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It does not do macro processing, include file handling, or
anything else you may get from your C compiler's preprocessor. You can
do include file processing with the .include directive
(see section .include). You can use the GNU C compiler driver
to get other "CPP" style preprocessing, by giving the input file a
`.S' suffix. See section `Options Controlling the Kind of Output' in Using GNU CC.
Excess whitespace, comments, and character constants cannot be used in the portions of the input text that are not preprocessed.
If the first line of an input file is #NO_APP or if you use the
`-f' option, whitespace and comments are not removed from the input file.
Within an input file, you can ask for whitespace and comment removal in
specific portions of the by putting a line that says #APP before the
text that may contain whitespace or comments, and putting a line that says
#NO_APP after this text. This feature is mainly intend to support
asm statements in compilers whose output is otherwise free of comments
and whitespace.
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Whitespace is one or more blanks or tabs, in any order. Whitespace is used to separate symbols, and to make programs neater for people to read. Unless within character constants (see section Character Constants), any whitespace means the same as exactly one space.
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There are two ways of rendering comments to as. In both
cases the comment is equivalent to one space.
Anything from `/*' through the next `*/' is a comment. This means you may not nest these comments.
/*
The only way to include a newline ('\n') in a comment
is to use this sort of comment.
*/
/* This sort of comment does not nest. */
|
Anything from the line comment character to the next newline is considered a comment and is ignored. The line comment character is `@' on the ARM; `#' on the i386 and x86-64; `!' on the SPARC; `#' on the 68HC11 and 68HC12; see 8. Machine Dependent Features.
On some machines there are two different line comment characters. One character only begins a comment if it is the first non-whitespace character on a line, while the other always begins a comment.
To be compatible with past assemblers, lines that begin with `#' have a special interpretation. Following the `#' should be an absolute expression (see section 6. Expressions): the logical line number of the next line. Then a string (see section Strings) is allowed: if present it is a new logical file name. The rest of the line, if any, should be whitespace.
If the first non-whitespace characters on the line are not numeric, the line is ignored. (Just like a comment.)
# This is an ordinary comment.
# 42-6 "new_file_name" # New logical file name
# This is logical line # 36.
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as.
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A symbol is one or more characters chosen from the set of all
letters (both upper and lower case), digits and the three characters
`_.$'.
On most machines, you can also use $ in symbol names; exceptions
are noted in 8. Machine Dependent Features.
No symbol may begin with a digit. Case is significant.
There is no length limit: all characters are significant. Symbols are
delimited by characters not in that set, or by the beginning of a file
(since the source program must end with a newline, the end of a file is
not a possible symbol delimiter). See section 5. Symbols.
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A statement ends at a newline character (`\n') or line separator character. (The line separator is usually `;', unless this conflicts with the comment character; see section 8. Machine Dependent Features.) The newline or separator character is considered part of the preceding statement. Newlines and separators within character constants are an exception: they do not end statements.
It is an error to end any statement with end-of-file: the last character of any input file should be a newline.
An empty statement is allowed, and may include whitespace. It is ignored.
A statement begins with zero or more labels, optionally followed by a
key symbol which determines what kind of statement it is. The key
symbol determines the syntax of the rest of the statement. If the
symbol begins with a dot `.' then the statement is an assembler
directive: typically valid for any computer. If the symbol begins with
a letter the statement is an assembly language instruction: it
assembles into a machine language instruction.
Different versions of as for different computers
recognize different instructions. In fact, the same symbol may
represent a different instruction in a different computer's assembly
language.
A label is a symbol immediately followed by a colon (:).
Whitespace before a label or after a colon is permitted, but you may not
have whitespace between a label's symbol and its colon. See section 5.1 Labels.
label: .directive followed by something
another_label: # This is an empty statement.
instruction operand_1, operand_2, ...
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A constant is a number, written so that its value is known by inspection, without knowing any context. Like this:
.byte 74, 0112, 092, 0x4A, 0X4a, 'J, '\J # All the same value. .ascii "Ring the bell\7" # A string constant. .octa 0x123456789abcdef0123456789ABCDEF0 # A bignum. .float 0f-314159265358979323846264338327\ 95028841971.693993751E-40 # - pi, a flonum. |
3.6.1 Character Constants 3.6.2 Number Constants
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There are two kinds of character constants. A character stands for one character in one byte and its value may be used in numeric expressions. String constants (properly called string literals) are potentially many bytes and their values may not be used in arithmetic expressions.
3.6.1.1 Strings 3.6.1.2 Characters
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A string is written between double-quotes. It may contain
double-quotes or null characters. The way to get special characters
into a string is to escape these characters: precede them with
a backslash `\' character. For example `\\' represents
one backslash: the first \ is an escape which tells
as to interpret the second character literally as a backslash
(which prevents as from recognizing the second \ as an
escape character). The complete list of escapes follows.
\008 has the value 010, and \009 the value 011.
x hex-digits...
x works.
as has no
other interpretation, so as knows it is giving you the wrong
code and warns you of the fact.
Which characters are escapable, and what those escapes represent, varies widely among assemblers. The current set is what we think the BSD 4.2 assembler recognizes, and is a subset of what most C compilers recognize. If you are in doubt, do not use an escape sequence.
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A single character may be written as a single quote immediately
followed by that character. The same escapes apply to characters as
to strings. So if you want to write the character backslash, you
must write '\\ where the first \ escapes the second
\. As you can see, the quote is an acute accent, not a
grave accent. A newline
immediately following an acute accent is taken as a literal character
and does not count as the end of a statement. The value of a character
constant in a numeric expression is the machine's byte-wide code for
that character. as assumes your character code is ASCII:
'A means 65, 'B means 66, and so on.
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as distinguishes three kinds of numbers according to how they
are stored in the target machine. Integers are numbers that
would fit into an int in the C language. Bignums are
integers, but they are stored in more than 32 bits. Flonums
are floating point numbers, described below.
3.6.2.1 Integers 3.6.2.2 Bignums 3.6.2.3 Flonums
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A binary integer is `0b' or `0B' followed by zero or more of the binary digits `01'.
An octal integer is `0' followed by zero or more of the octal digits (`01234567').
A decimal integer starts with a non-zero digit followed by zero or more digits (`0123456789').
A hexadecimal integer is `0x' or `0X' followed by one or more hexadecimal digits chosen from `0123456789abcdefABCDEF'.
Integers have the usual values. To denote a negative integer, use the prefix operator `-' discussed under expressions (see section Prefix Operators).
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A bignum has the same syntax and semantics as an integer except that the number (or its negative) takes more than 32 bits to represent in binary. The distinction is made because in some places integers are permitted while bignums are not.
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A flonum represents a floating point number. The translation is
indirect: a decimal floating point number from the text is converted by
as to a generic binary floating point number of more than
sufficient precision. This generic floating point number is converted
to a particular computer's floating point format (or formats) by a
portion of as specialized to that computer.
A flonum is written by writing (in order)
as the rest of the number is a flonum.
e is recommended. Case is not important.
On the H8/300, H8/500, Hitachi SH, and AMD 29K architectures, the letter must be one of the letters `DFPRSX' (in upper or lower case).
On the ARC, the letter must be one of the letters `DFRS' (in upper or lower case).
On the Intel 960 architecture, the letter must be one of the letters `DFT' (in upper or lower case).
On the HPPA architecture, the letter must be `E' (upper case only).
At least one of the integer part or the fractional part must be present. The floating point number has the usual base-10 value.
as does all processing using integers. Flonums are computed
independently of any floating point hardware in the computer running
as.
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4.1 Background 4.2 Linker Sections 4.3 Assembler Internal Sections 4.4 Sub-Sections 4.5 bss Section
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Roughly, a section is a range of addresses, with no gaps; all data "in" those addresses is treated the same for some particular purpose. For example there may be a "read only" section.
The linker ld reads many object files (partial programs) and
combines their contents to form a runnable program. When as
emits an object file, the partial program is assumed to start at address 0.
ld assigns the final addresses for the partial program, so that
different partial programs do not overlap. This is actually an
oversimplification, but it suffices to explain how as uses
sections.
ld moves blocks of bytes of your program to their run-time
addresses. These blocks slide to their run-time addresses as rigid
units; their length does not change and neither does the order of bytes
within them. Such a rigid unit is called a section. Assigning
run-time addresses to sections is called relocation. It includes
the task of adjusting mentions of object-file addresses so they refer to
the proper run-time addresses.
An object file written by as has at least three sections, any
of which may be empty. These are named text, data and
bss sections.
as can also generate whatever other named sections you specify
using the `.section' directive (see section .section).
If you do not use any directives that place output in the `.text'
or `.data' sections, these sections still exist, but are empty.
as can also generate whatever other named sections you
specify using the `.space' and `.subspace' directives. See
HP9000 Series 800 Assembly Language Reference Manual
(HP 92432-90001) for details on the `.space' and `.subspace'
assembler directives.
Within the object file, the text section starts at address 0, the
data section follows, and the bss section follows the data section.
To let ld know which data changes when the sections are
relocated, and how to change that data, as also writes to the
object file details of the relocation needed. To perform relocation
ld must know, each time an address in the object
file is mentioned:
(address) - (start-address of section)? |
In fact, every address as ever uses is expressed as
(section) + (offset into section) |
as computes have this section-relative
nature.
In this manual we use the notation {secname N} to mean "offset N into section secname."
Apart from text, data and bss sections you need to know about the
absolute section. When ld mixes partial programs,
addresses in the absolute section remain unchanged. For example, address
{absolute 0} is "relocated" to run-time address 0 by
ld. Although the linker never arranges two partial programs'
data sections with overlapping addresses after linking, by definition
their absolute sections must overlap. Address {absolute 239} in one
part of a program is always the same address when the program is running as
address {absolute 239} in any other part of the program.
The idea of sections is extended to the undefined section. Any address whose section is unknown at assembly time is by definition rendered {undefined U}---where U is filled in later. Since numbers are always defined, the only way to generate an undefined address is to mention an undefined symbol. A reference to a named common block would be such a symbol: its value is unknown at assembly time so it has section undefined.
By analogy the word section is used to describe groups of sections in
the linked program. ld puts all partial programs' text
sections in contiguous addresses in the linked program. It is
customary to refer to the text section of a program, meaning all
the addresses of all partial programs' text sections. Likewise for
data and bss sections.
Some sections are manipulated by ld; others are invented for
use of as and have no meaning except during assembly.
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ld deals with just four kinds of sections, summarized below.
as and ld treat them as
separate but equal sections. Anything you can say of one section is
true another.
When the program is running, however, it is
customary for the text section to be unalterable. The
text section is often shared among processes: it contains
instructions, constants and the like. The data section of a running
program is usually alterable: for example, C variables would be stored
in the data section.
ld must
not change when relocating. In this sense we speak of absolute
addresses being "unrelocatable": they do not change during relocation.
An idealized example of three relocatable sections follows. Memory addresses are on the horizontal axis.
+-----+----+--+
partial program # 1: |ttttt|dddd|00|
+-----+----+--+
text data bss
seg. seg. seg.
+---+---+---+
partial program # 2: |TTT|DDD|000|
+---+---+---+
+--+---+-----+--+----+---+-----+~~
linked program: | |TTT|ttttt| |dddd|DDD|00000|
+--+---+-----+--+----+---+-----+~~
addresses: 0 ...
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These sections are meant only for the internal use of as. They
have no meaning at run-time. You do not really need to know about these
sections for most purposes; but they can be mentioned in as
warning messages, so it might be helpful to have an idea of their
meanings to as. These sections are used to permit the
value of every expression in your assembly language program to be a
section-relative address.
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Assembled bytes
fall into two sections: text and data.
You may have separate groups of
data in named sections
text or data
that you want to end up near to each other in the object file, even though they
are not contiguous in the assembler source. as allows you to
use subsections for this purpose. Within each section, there can be
numbered subsections with values from 0 to 8192. Objects assembled into the
same subsection go into the object file together with other objects in the same
subsection. For example, a compiler might want to store constants in the text
section, but might not want to have them interspersed with the program being
assembled. In this case, the compiler could issue a `.text 0' before each
section of code being output, and a `.text 1' before each group of
constants being output.
Subsections are optional. If you do not use subsections, everything goes in subsection number zero.
Each subsection is zero-padded up to a multiple of four bytes.
(Subsections may be padded a different amount on different flavors
of as.)
Subsections appear in your object file in numeric order, lowest numbered
to highest. (All this to be compatible with other people's assemblers.)
The object file contains no representation of subsections; ld and
other programs that manipulate object files see no trace of them.
They just see all your text subsections as a text section, and all your
data subsections as a data section.
To specify which subsection you want subsequent statements assembled
into, use a numeric argument to specify it, in a `.text
expression' or a `.data expression' statement.
can also use an extra subsection
argument with arbitrary named sections: `.section name,
expression'.
Expression should be an absolute expression.
(See section 6. Expressions.) If you just say `.text' then `.text 0'
is assumed. Likewise `.data' means `.data 0'. Assembly
begins in text 0. For instance:
.text 0 # The default subsection is text 0 anyway. .ascii "This lives in the first text subsection. *" .text 1 .ascii "But this lives in the second text subsection." .data 0 .ascii "This lives in the data section," .ascii "in the first data subsection." .text 0 .ascii "This lives in the first text section," .ascii "immediately following the asterisk (*)." |
Each section has a location counter incremented by one for every byte
assembled into that section. Because subsections are merely a convenience
restricted to as there is no concept of a subsection location
counter. There is no way to directly manipulate a location counter--but the
.align directive changes it, and any label definition captures its
current value. The location counter of the section where statements are being
assembled is said to be the active location counter.
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The bss section is used for local common variable storage. You may allocate address space in the bss section, but you may not dictate data to load into it before your program executes. When your program starts running, all the contents of the bss section are zeroed bytes.
The .lcomm pseudo-op defines a symbol in the bss section; see
.lcomm.
The .comm pseudo-op may be used to declare a common symbol, which is
another form of uninitialized symbol; see See section .comm.
When assembling for a target which supports multiple sections, such as ELF or
COFF, you may switch into the .bss section and define symbols as usual;
see .section. You may only assemble zero values into the
section. Typically the section will only contain symbol definitions and
.skip directives (see section .skip).
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Symbols are a central concept: the programmer uses symbols to name things, the linker uses symbols to link, and the debugger uses symbols to debug.
Warning: as does not place symbols in the object file in
the same order they were declared. This may break some debuggers.
5.1 Labels 5.2 Giving Symbols Other Values 5.3 Symbol Names 5.4 The Special Dot Symbol 5.5 Symbol Attributes
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A label is written as a symbol immediately followed by a colon `:'. The symbol then represents the current value of the active location counter, and is, for example, a suitable instruction operand. You are warned if you use the same symbol to represent two different locations: the first definition overrides any other definitions.
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A symbol can be given an arbitrary value by writing a symbol, followed
by an equals sign `=', followed by an expression
(see section 6. Expressions). This is equivalent to using the .set
directive. See section .set.
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Symbol names begin with a letter or with one of `._'. On most
machines, you can also use $ in symbol names; exceptions are
noted in 8. Machine Dependent Features. That character may be followed by any
string of digits, letters, dollar signs (unless otherwise noted in
8. Machine Dependent Features), and underscores.
Case of letters is significant: foo is a different symbol name
than Foo.
Each symbol has exactly one name. Each name in an assembly language program refers to exactly one symbol. You may use that symbol name any number of times in a program.
Local symbols help compilers and programmers use names temporarily. There are ten local symbol names, which are re-used throughout the program. You may refer to them using the names `0' `1' ... `9'. To define a local symbol, write a label of the form `N:' (where N represents any digit). To refer to the most recent previous definition of that symbol write `Nb', using the same digit as when you defined the label. To refer to the next definition of a local label, write `Nf'---where N gives you a choice of 10 forward references. The `b' stands for "backwards" and the `f' stands for "forwards".
Local symbols are not emitted by the current GNU C compiler.
There is no restriction on how you can use these labels, but remember that at any point in the assembly you can refer to at most 10 prior local labels and to at most 10 forward local labels.
Local symbol names are only a notation device. They are immediately transformed into more conventional symbol names before the assembler uses them. The symbol names stored in the symbol table, appearing in error messages and optionally emitted to the object file have these parts:
L
as and
ld forget symbols that start with `L'. These labels are
used for symbols you are never intended